Every interferometer relies on the principle of superimposing an internal reference beam on a coherent light beam reflected from the target and analysing the resulting interference signal.
The interference signal changes in different ways, depending on which parameters of the cavity change. In the case of length changes of the interferometer (of the cavity) or due to wavelength changes of the coherent light source, there is a sweep-through of the phase of the periodic (sine- or cosine-shaped) interference signal. The speed of this phase change is proportional to the speed of a length change of the cavity or the speed of a wavelength change of the light source multiplied by the current length of the cavity. In the case of changes to other optical parameters of the cavity, e.g. the contrast or the amplitude of the interference signal change.
From patent application EP 2 363 685 A1 there is known a device and a method for interferometric position acquisition, which exhibits a Fabry-Perot interferometer. By measuring the intensity of the light reflected from the mirrors of the Fabry-Perot interferometer it is possible to ascertain the mirror separation and/or a change in the mirror separation. Since the wavelength λ of the measurement light is known, it is possible to determine the change in the length of the resonator of the Fabry-Perot interferometer from a measurement of the intensity of the reflected light.
Methods for distance measurement such as the one described in EP 2 363 685 A1 attempt to keep the wavelength known and constant, so that an unambiguous inference from changes in the sinusoidal interference pattern to length changes of the cavity and thus changes in the distance of an object is possible (see also FIG. 1 and FIG. 2 in EP 2 363 685 A1).
It is problematic here that it requires great investment in resources to calibrate the wavelength and keep it constant. In addition a problem arises in the vicinity of the extreme points of the sinus, since there the dependence of the signal on the movement of the object is weaker. At the extreme points themselves it is only of second order and thus makes the signal ambiguous. Furthermore one cannot distinguish in this manner between changes in the length of the cavity and changes in other optical parameters such as, for example, the reflectivity and/or the index of refraction.
Two possibilities have been proposed and realised in the state of the art for solving part of these problems:                If several light sources (preferably lasers) with different wavelengths are used simultaneously, the problem of the extreme points occurs only very rarely in all light sources simultaneously. Thus they can be used alternately for measurement. Several lasers, however, mean at the same time greatly increased investment in resources and thus increased costs.        Through high-frequency modulation of the wavelength of the tuneable lasers at a small amplitude, which in effect corresponds to a small movement of the target, in addition to the interference signal its approximated first derivative by position is measured. This is complementary to the direct signal in the sense that its strongest dependence on the position exists exactly where the direct signal is insensitive—and vice versa. Together they always yield an analysable signal. Since the ‘derivation’ is based on forming a difference, however, it amplifies the noise of the interference signal considerably.        
The disadvantage of these possibilities, then, is that they either increase the investment in resources and/or utilise further information that suffers from additional measurement inaccuracy. Furthermore in this method it is not possible or possible only with great difficulty to infer from a change in the interference signal directly to a change in the length of the cavity. Optical conditions could also always have changed, which then lead to an error in the length measurement. A further problem is that in the case of a still-standing object and a fixedly adjusted wavelength, the interference signal also hardly changes and therefore a very high sensitivity to disturbances exists.
The task of this invention, therefore, is to create a method and a device for interferometric distance measurement which solve the problems known from the state of the art, implement them at a comparatively low investment in resources and in particular make possible the highest possible measurement accuracy.